Upgrading an instance to Madrid does not retain customizations previously done on the Banner Image.
Steps to Reproduce
1. Login a London instance as administrator.
2. Navigate to Basic Ui16 configuration.
3. Upload a new Banner Image.
4. Upgrade the instance to Madrid and wait for completion.
Notice the Banner Image will not retain the previous customizations and will default back to the OOB "ServiceNow" banner image.
This problem is under review and targeted to be fixed in a future release. You can Subscribe to this article to receive notifications when more information will become available.
As a workaround, repeat the customization to override the default banner logo with a new image on My Company:
Related Problem: PRB1319296